Segregated catalyzed metallic wire filter for diesel soot filtration

ABSTRACT

A filter for removing soot particles from the exhaust gas of a diesel engine comprises a plurality of hollow channels which contain therein a metal mesh. The metal mesh can be coated with an oxidation catalyst to promote ignition of the soot particles and regeneration of the metal mesh for filtering the soot particles. The metal mesh can optionally be removed from the hollow channels and replaced with regenerated or new metal mesh if desired.

FIELD OF THE INVENTION

This invention relates to diesel engine exhaust gas treatment and moreparticularly to the filtering of particulates from diesel engine exhaustgases using a catalyzed filter.

BACKGROUND OF THE INVENTION

Diesel engine exhaust is a heterogeneous mixture which contains not onlygaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons(“HC”) and nitrogen oxides (“NOx”), but also condensed phase materials(liquids and solids) which constitute the so-called particulates orparticulate matter (“PM”). The total particulate matter (“TPM”)emissions are comprised of three main components. One component is thesolid, dry, solid carbonaceous fraction or soot. This dry carbonaceousmatter contributes to the visible soot emissions commonly associatedwith diesel exhaust. A second component of the TPM is the solubleorganic fraction (“SOF”). The soluble organic fraction is sometimesreferred to as the volatile organic fraction (“VOF”), which terminologywill be used herein. The VOF may exist in diesel exhaust either as avapor or as an aerosol (fine droplets of liquid condensate) depending onthe temperature of the diesel exhaust, and are generally present ascondensed liquids at the standard particulate collection temperature of52° C. in diluted exhaust, as prescribed by a standard measurement test,such as the U.S. Heavy Duty Transient Federal Test Procedure. Theseliquids arise from two sources: (1) lubricating oil swept from thecylinder walls of the engine each time the pistons go up and down; and(2) unburned or partially burned diesel fuel.

The third component of the particulates is the so-called sulfatefraction. Diesel fuel contains sulfur, and even the low sulfur fuelavailable in the U.S. may contain 0.005% sulfur. Upon combustion of thefuel in the engine, nearly all of the sulfur is oxidized to sulfurdioxide which exits with the exhaust in the gas phase. However, a smallportion of the sulfur, perhaps 2-5%, is oxidized further to SO₃, whichin turn combines rapidly with water in the exhaust to form sulfuric acidwhich collects as a condensed phase with the particulates as an aerosol,or is adsorbed onto the other particulate components, and thereby addsto the mass of TPM.

Emissions from diesel engines have been under increasing scrutiny inrecent years and standards, especially for particulate emissions, havebecome stricter. In 1994 the particulate emission standards in the U.S.for new engines allowed no more than a total of 0.1 grams per brakehorse power hour (g/BHP-h). For diesel engines in buses operating incongested urban areas the particulate emissions standard was evenstricter, 0.07 g/BHP-h TPM. Both of these standards were seen assignificant reductions relative to the prior particulate emissionstandard of 0.25 g/BHP-h which had been in effect since 1991. Startingin 1994, for the first time, engine technology developments alone werefound to be incapable of meeting the new standards, and for some enginesafter treatment technology, for example, diesel oxidation catalyst (DOC)units, as discussed further below, were necessary.

The question of how best to reduce the levels of particulate matterexpelled to the atmosphere in the exhaust gases of diesel engines iscurrently of considerable interest as stricter emission standards areconstantly being legislated through the next decade. In this connection,it is desired to develop efficient and practical devices for removingsubstantial portions of particulates from the exhaust gases in dieselengine exhaust systems before permitting the exhaust gases to escape tothe atmosphere.

It is known in the art to provide diesel engines with an exhaust filterwhich traps particulates from the exhaust gas stream during engineoperation. The filters are generally made of porous, solid materialshaving a plurality of pores extending therethrough and having smallcross-sectional size, such that the filter is permeable to the exhaustgases which flow through the filters and are capable of restraining mostor all of the particulates from passing through the filter with the gas.The restrained particulates consist generally of carbonaceousparticulates in the form of soot particles and reference herein and inthe claims to “particulate” and “particulates” means such dieselengine-generated particles. As the mass of collected particulatesincreases, the flow rate of the exhaust gas through the filter isusually impeded, whereby an increased back pressure is encounteredwithin the filter and reduced engine efficiency results.

There is a desire in the art to more simply regenerate the particulatefilter by continuous burn-off or incineration of the soot particles asthey are trapped in the filter. However, experience has shown that innormal diesel engine operation, the temperature in the exhaust systemvaries substantially under different conditions of engine load and speedand that the temperatures in the filter hardly ever reach the 510° C.temperature level required to incinerate the trapped particulate.

In order to comply with the ever-increasing legislation both in theUnited States and Europe to reduce the level of solid emissions fromboth on- and off-highway diesel-powered vehicles, exhaustafter-treatment, such as a variety of soot filter media, have beenexplored. The wallflow type ceramic honeycomb filter is the most widelyemployed filtration technology used in current systems for industrialapplications. Wallflow filters provide an answer to the filtrationrequirement, yet there remains the residual problem of achieving areliable and repeatable method of cleaning the filter. This residualproblem has been the source of extensive engineering research anddevelopment. Wallflow filter elements are particularly useful to filterparticulate matter from diesel engine exhaust gases. Many referencesdisclose the use of wallflow filters which can comprise catalysts on orin the filter to filter and burn off filtered particulate matter. Acommon ceramic wallflow filter construction is a multi-channel honeycombstructure having the ends of alternate channels on the upstream anddownstream sides of the honeycomb structure plugged. This results in acheckerboard-type pattern on either end. Channels plugged on theupstream or inlet end are open on the downstream or outlet end. Thispermits the gas to enter the open upstream channels, flow through theporous walls and exit through the channels having open downstream ends.The gas pressure forces the gas through the porous structural walls intothe channels closed at the upstream end and open at the downstream end.Such structures are primarily disclosed to filter particles out of theexhaust gas stream.

It is desired to remove the particulate matter from the upstream sidesof the wallflow filters. One method is to provide a layer of catalyst onthe wall to catalyze the ignition of the particulate matter duringoperation of the filter. There are many U.S. patents disclosing suchwallflow structures.

A particularly useful particulate emission control filter directed foruse for diesel exhaust is presented in “3M Diesel Filters forParticulate Emission Control, Designers Guide” published by 3M CeramicMaterials Department, printed 1994 January and hereby incorporated byreference. There is described a ceramic filter comprising ceramic fiberspecified to have 62% Al₂O₃, 24% SiO₂, and 14% B₂O₃. The filterspecification includes a white continuous fiber having a fiber diameterof 10-12 microns with a fiber density of 2.7 grams per cubic centimeter.The mechanical properties of the fiber include a filament tensilestrength of 1.72 GPA, a filament tensile modulus of elasticity of 138GPA, and elongation of 1.2%. The specified thermal properties arecontinuous use temperature of 1204° C., short-term use temperature at1371° C., a lineal shrinkage at 1093° C. of 1.25%, a melting point of1800° C., a thermal expansion co-efficient (25-500° C.) of 3.0×10⁻⁶ΔL/L° C., and a specific heat of 1046.7 J/Kg° K. The fiber is sold bythe 3M Ceramic Materials Department as NEXTEL™ FIBER. The abovespecified properties are for NEXTEL™ 312 CERAMIC FIBER.

The NEXTEL™ fibers are used to make diesel filters. A typical 3M dieselfilter cartridge has a cylindrical support and a continuous ceramicfiber woven in a diamond pattern on the support to form a ceramic fiberwinding, see U.S. Pat. No. 5,551,971, FIG. 1. The cylindrical support isan electric resistant heating element that contains openings. The areaof the openings can be used to control the heat input along the support.Where less heat is desired, the support can have larger openings or moreopenings at a given location. The distribution of openings can be variedwith the most open area toward the center of the support. Thecylindrical support has an open end and a closed end. The filter isuseful to filter particulate matter from diesel engine exhaust. Duringengine operation gas laden with particulate matter can pass through theouter circumferential surface of the ceramic fiber windings through theopen areas of the cylindrical support and out through the open end.Alternatively and preferably, the filter cartridge can be operated inreverse. Particle laden gases can be fed into open end, pass through theopen areas of cylindrical support and then through ceramic fiberwindings depositing its particles in the ceramic fiber windings.

During heating to regenerate the filter, an oxygen laden gas, preferablyair, is fed into open end. Electric energy is input to heat thecylindrical support which acts as a heating element or heater. Thecylindrical heating element heats the ceramic fiber windings to atemperature sufficient to oxidize particulate matter trapped thereon.

The filter cartridge is used in a diesel engine exhaust system.Typically, a plurality of filters are assembled within a canister. Thenumber of filter cartridges assembled in a canister is sized to theexhaust flow rates and anticipated regeneration intervals.

While a variety of soot filters are known in the art, improvements arecontinually desired not only in the regeneration of such filters, butfor the ease of manufacture, retrofitting, and replacement of suchfilters. Improvements are further desired in maintaining gas flowthrough the filters even if soot accumulation exceeds the soot-burningrate of the filtering media so as to keep the vehicle running untilcleaning can occur.

SUMMARY OF THE INVENTION

A diesel soot filter is provided comprising a plurality of parallelchannels that are composed of a metallic mesh to trap soot particles asthe diesel exhaust gas passes through the channels. The filteringchannels can be arranged such as in a canister such that smaller by-passgas channels are formed between the filtering channels. The by-passchannels allow the exhaust gas to pass therethrough in the event thatsoot accumulation in the main filtering channels exceeds the burningrate of the accumulated soot. The reduced gas flow through the by-passchannels reduces the back pressure and keeps the vehicle running until afavorable regeneration of the filter is achieved. The wire mesh can beremoved from the channels and replaced with new metal mesh, and/or thechannels containing the metal mesh may also be removed and replaced ifneeded to renew soot removal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal cross-sectional view of a canister holding thesoot filter channels of this invention and taken along line 1-1 of FIG.2.

FIG. 2 is a transverse sectional view of the canister and soot filterdevice of the present invention taken along line 2-2 of FIG. 1.

FIG. 3 is a perspective view of a soot filter channel of this invention.

FIG. 4 is a plan view of one of the end plates which can be used to holdthe soot filter channels in place.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, the filtering element of this invention is shown installed asa diesel particulate trap 10 which has a canister of rectangular tubularcasing 12, a pyramidal exhaust inlet 14, and a pyramidal exhaust outlet16. As installed, a plurality of hollow channels 18 extend in the axialor longitudinal direction of the filtering element which is also theprimary direction of the flow of exhaust through the diesel particulatetrap. The metal sleeve 20 of the filtering element has been sealed tothe casing 12 by an intumescent mat 24 that expands when exposed to theheat of the first use of the diesel particulate trap. Any such matshould be selected to withstand temperatures encountered in use,especially temperatures at which the filtering element is to beregenerated. A particularly useful intumescent mat is provided by aheat-expandable vermiculite mat.

The channel walls which form each of channels 18 are impervious metalsheet in circular, square, trapezoid, rectangular, etc., designs shownin FIGS. 1 and 2 as cylindrical channels 18 having a circularcross-section and open at each end 19 and 21. The inside diameter ofchannel 18, regardless of shape, will be at least 0.5 in, preferably atleast about 1.0 inch wide. A plurality of channels 18 are placed withinenclosed area 26 of casing 12. End plates 23 and 25 on opposite ends ofarea 26 and welded or otherwise attached to metal sleeve 20 support theopposing ends of channels 18. End plates 23 and 25 can include openingsthrough which the ends of channels 18 are supported. The channels 18 maybe permanently fixed to end plates 23 and 25 or temporarily fit so thatsuch channels can be replaced due to wear. Thus, the outside diametersor perimeters of channels 18 can be such as to be pressure fit withinthe openings 32 of the end plates 23 and 25, see FIG. 4, or otherwiseremovably attached thereto such by bolts, screws, etc., so as to remainin place during use. Upon wear, the channels 18 can be removed from therespective openings 32 in the end plates 23 and 25 and slid from theinterior of enclosed area 26 for repair and/or replacement.

Within the interior of each channel 18 is placed a metal mesh filteringelement 28 which is capable of trapping the soot particles containedwithin the diesel exhaust. The metal mesh can be of various types andconfigurations so long as the mesh filtering element 28 allows for gasflow therethrough, but forms a barrier for soot particles. Woven metalmesh such as of steel wool type, non-woven wire mesh in which individualwires are spot soldered or the like with other wires to form a singlemesh unit, braided wire mesh in which a plurality of wire strands aretwisted together to form a mesh capable of the desired removal of sootfrom a passing gas stream can be used. The metal mesh filtering element28 is preferably placed as a single mass within the interior of eachchannel 18 so as to substantially completely fill the channel interior.More than one piece of metal mesh can be used if more convenient to fillthe channel interiors. The metal mesh filtering element 28 can bereadily removable from within each channel 18 so as to be easilysubstituted with repaired, regenerated, or a new mesh element tomaintain optimum filtering capability. The metal mesh filtering element28 can be pressure fit within each channel interior so as to remain inplace during operation. Alternatively, or in addition to pressurefitting, each open end 19 and 21 of channel 18 may contain one or morecross pieces or mesh (not shown) to keep the metal mesh filteringelement 28 in place within the interior of channels 18.

It is well known in the art of diesel soot filters to burn off the sootparticles from the filter so as to regenerate the filter and againimprove its capacity to filter the soot particles from the exhaust gas.Unfortunately, the temperature generated by the diesel engine andimparted to the exhaust gas is not high enough to initiate ignition andburning of the soot particles. Accordingly, oxidation catalysts havebeen incorporated onto the filtering element so as to lower the ignitiontemperature of the soot particles and allow the particles to be burnedand the filter element regenerated either on a continuous or alternatingprocess between filtering and regenerating. In accordance with thepresent invention, the metal mesh elements 28 of this invention can becoated with the oxidation catalysts well known in the art to initiatethe ignition of the soot particles from the diesel exhaust gas that aretrapped within the filtering element. The types of catalysts and themethods of applying the catalysts to the metal mesh filtering element 28are more fully explained below. In addition to coating the wire meshfiltering element 28, it is also possible to coat the interior of thechannels 18 with an oxidation catalyst to initiate ignition of any sootparticles attached to the interior walls and/or initiate oxidation ofgaseous contaminants within the exhaust gas itself, such as CO, HC, andNOx. The catalyst on the metal mesh filtering element 28 may be the sameor different than the catalyst that is coated on the interior walls ofchannels 18. Further, any wire elements used to maintain the filteringelements 28 in place within channels 18, such as any mesh or the like,provided in openings 19 and 21 can also be provided with a catalyticcoating. Still further, it may be possible to provide an oxidationcatalyst on at least a portion of the exterior of the channels 18, inparticular, those areas of the channels which are in contact with theinterstitial voids 30, which are formed between the channels 18 as thechannels are stacked within enclosed area 26 of casing 12. Thus, exhaustgas that passes through the interstitial voids 30 can be treated so asto initiate oxidation of the exhaust gas contaminants.

The filter device 10 of this invention will be placed in an exhauststream from a diesel engine. A diesel oxidation catalyst (DOC) may ormay not be placed in front of the filtering device 10 dependent upon theapplication. An exhaust gas from the diesel engine containing HC, CO,NOx, and particulate matter passes through the filter device 10 and, inparticular, channels 18. Due to the impaction of the soot particles onthe catalyzed wire mesh element 28, the soot particles are collected andburnt under suitable exhaust regeneration conditions. If the applicationis such that the soot accumulation rate on filter element 28 exceeds theburning rate of the soot particles, exhaust gas flow from the enginewill be forced and diverted through the interstices 30 between thestacked channels 18 within casing 12. The interstitial voids 30 areinitially sized to permit only minor flow during most diesel engineoperating conditions since the back pressure in the interstities 30 ishigher than the back pressure through mesh filtering element 28.Typically the interstitial void volume within enclosed area 26 of casing12 will comprise less than 25% of the volume of enclosed area 26. Gasflow through the interstitial void volume can be controlled or limitedby use of orifice openings 34 in the endplates such as shown forendplate 23 in FIG. 4. When the soot accumulates and starts to block thepores of the metal mesh filtering element 28 and the back pressurerises, the interstitial or bypass voids 30 still enable exhaust gas flowand allow the diesel engine to continue operation. In this way thevehicle will not stall due to a totally restricted flow path of theexhaust gas. The density of the wire mesh (wire diameter and the amountof wire weaved into the matrix) determines the back pressure.

The metal mesh can be made of any relatively high temperature alloy,including most stainless steels, Fecralloy, Hastalloy, etc.

In a preferred embodiment of the invention, the metal mesh is pretreatedprior to deposition of the catalyst composition to improve the adherenceof composition on the substrate. Pretreatment of the substrate can beconducted by applying a metal anchor layer to the substrate by knownthermal spraying techniques before the catalyst slurry is applied. Thesetechniques include plasma spraying, single wire spraying, high velocityoxy-fuel spraying, combustion wire and/or powder spraying, electric arcspraying etc. Preferably the metal anchor layer is applied by electricarc spraying.

Electric arc spraying, e.g., twin wire arc spraying, of a metal (whichterm, as used herein, includes mixtures of metals, including withoutlimitation, metal alloys, pseudoalloys, and other intermetalliccombinations) onto a metal foraminous substrate yields a structurehaving superior utility as a substrate for catalytic materials in thefield of catalyst members. Twin wire arc spraying (encompassed herein bythe term “wire arc spraying” and by the broader term “electric arcspraying”) is a known process, disclosed in U.S. Pat. No. 4,027,367which is incorporated herein by reference. Briefly described, in thetwin wire arc spray process, two feedstock wires act as two consumableelectrodes. These wires are insulated from each other as they are fed tothe spray nozzle of a spray gun in a fashion similar to wire flame guns.The wires meet in the center of a gas stream generated in the nozzle. Anelectric arc is initiated between the wires, and the current flowingthrough the wires causes their tips to melt. A compressed atomizing gas,usually air, is directed through the nozzle and across the arc zone,shearing off the molten droplets to form a spray that is propelled ontothe substrate. Only metal wire feedstock can be used in an arc spraysystem because the feedstock must be conductive. The high particletemperatures created by the spray gun produce minute weld zones at theimpact point on a metallic substrate. As a result, such electric arcspray coatings (sometimes referred to herein as “anchor layers”)maintain a strong adhesive bond with the substrate.

Operating parameters for wire arc spraying for forming anchor layer onforaminous substrates are disclosed in copending U.S. patent applicationSer. No. 09/301,626, filed Apr. 29, 1999 (the '626 application), nowU.S. Publication No. 2002/0128151, published Sep. 12, 2002, thedisclosure of which is hereby incorporated by reference in its entirety.

Anchor layers of a variety of compositions can be deposited on asubstrate by utilizing, without limitation, feedstocks of the followingmetals and metal mixtures: Ni, Ni/Al, Ni/Cr, Ni/Cr/Al/Y, Co/Cr,Co/Cr/Al/Y, Co/Ni/Cr/Al/Y, Fe/Al, Fe/Cr, Fe/Cr/Al, Fe/Cr/Al/Y, Fe/Ni/Al,Fe/Ni/Cr, 300 and 400 series stainless steels, and, optionally, mixturesof one or more thereof. One specific example of a metal useful for wirearc spraying onto a substrate in accordance with the '626 application isa nickel/aluminum alloy that generally contains at least about 90%nickel and from about 3% to 10% aluminum, preferably from about 4% to 6%aluminum by weight. Such an alloy may contain minor proportions of othermetals referred to herein as “impurities” totaling not more than about2% of the alloy. A preferred specific feedstock alloy comprises about95% nickel and 5% aluminum and may have a melting point of about 2642°F. Some such impurities may be included in the alloy for variouspurposes, e.g., as processing aids to facilitate the wire arc sprayingprocess or the formation of the anchor layer, or to provide the anchorlayer with favorable properties.

Electric arc spraying a metal onto a metal substrate yields a superiorsubstrate for catalytic materials relative to substrates having metalanchor layers applied thereto by other methods. Catalytic materials havebeen seen to adhere better to a substrate comprising an electric arcsprayed anchor layers than to a substrate without an intermediate layerapplied thereto and even better than to a substrate having a metal layerdeposited thereon by plasma spraying. Catalytic materials disposed onmetal substrates, without intermediate layers between the substrate andthe catalytic material, often did not adhere sufficiently well to thesubstrate to provide a commercially acceptable product. Metal substrateshaving an intermediate layer applied by other thermal sprayingtechniques typically suffer the same drawbacks. For example, a metalsubstrate having a metal intermediate layer that was plasma-sprayedthereon and having a catalytic material applied to the intermediatelayer failed to retain the catalytic material, which flaked off uponroutine handling, apparently due to a failure of the intermediate layerto bond with the substrate. The catalytic material on other substrateswas seen to spall off upon normal use, apparently as a result of beingsubjected to a high gas flow rate, to thermal cycling, to the erodingcontact of high temperature steam and other components of the exhaustgas stream, vibrations, etc. Application of the intermediate layer byelectric arc spraying therefore improves the durability of catalystmembers comprising catalytic materials carried on foraminous substratesby improving their durability.

The metal mesh filter elements of this invention (also referred toherein as foraminous substrates) useful for forming the filteringelements include those metallic substrates which are able to accommodatea high flow rate, are lightweight and have a low thermal mass. Thewoven, non-woven, and braided wire mesh of this invention as filterelement 28 are suitable for application of a metal anchor layer.

A suitable catalytic material for use on a foraminous substrate can beprepared by dispersing a compound and/or complex of any catalyticallyactive component, e.g., one or more platinum group metal compounds orcomplexes, onto relatively inert bulk support material. As used herein,the term “compound”, as in “platinum group metal compound” means anysalt, complex, or the like of a catalytically active component (or“catalytic component”) which, upon calcination or upon use of thecatalyst, decomposes or otherwise converts to a catalytically activeform, which is often, but not necessarily, an oxide. The compounds orcomplexes of one or more catalytic compounds may be dissolved orsuspended in any liquid which will wet or impregnate the supportmaterial, which does not adversely react with other components of thecatalytic material and which is capable of being removed from thecatalyst by volatilization or decomposition upon heating and/or theapplication of a vacuum. Generally, both from the point of view ofeconomics and environmental aspects, aqueous solutions of solublecompounds or complexes are preferred. For example, suitablewater-soluble platinum group metal compounds are chloroplatinic acid,amine solubilized platinum hydroxide, rhodium chloride, rhodium nitrate,hexamine rhodium chloride, palladium nitrate or palladium chloride, etc.The compound-containing liquid is impregnated into the pores of the bulksupport particles of the catalyst, and the impregnated material is driedand preferably calcined to remove the liquid and bind the platinum groupmetal into the support material. In some cases, the completion ofremoval of the liquid (which may be present as, e.g., water ofcrystallization) may not occur until the catalyst is placed into use andsubjected to the high temperature exhaust gas. During the calcinationstep, or at least during the initial phase of use of the catalyst, suchcompounds are converted into a catalytically active form of the platinumgroup metal or a compound thereof. An analogous approach can be taken toincorporate the other components into the catalytic material.Optionally, the inert support materials may be omitted and the catalyticmaterial may consist essentially of the catalytic component depositeddirectly on the sprayed foraminous substrate by conventional methods.

Preferred platinum group metal components for use in the articles of theinvention include platinum, palladium, rhodium, ruthenium and iridiumcomponents. Platinum, palladium and rhodium components are particularlypreferred. When deposited on a foraminous substrate (e.g., metal screen)such components are generally deposited at a concentration of from 0.001to 0.01 g/in² for typical utility engine applications.

Suitable support materials for the catalytic component include alumina,silica, titania, silica-alumina, alumino-silicates, aluminum-zirconiumoxide, aluminum-chromium oxide, etc. Such materials are preferably usedin their high surface area forms. For example, gamma-alumina ispreferred over alpha-alumina. It is known to stabilize high surface areasupport materials by impregnating the material with a stabilizerspecies. For example, gamma-alumina can be stabilized against thermaldegradation by impregnating the material with a solution of a ceriumcompound and then calcining the impregnated material to remove thesolvent and convert the cerium compound to a cerium oxide. Thestabilizing species may be present in an amount of from about, e.g., 5percent by weight of the support material. The catalytic materials aretypically used in particulate form with particles in the micron-sizedrange, e.g., 10 to 20 microns in diameter, so that they can be formedinto a slurry and coated onto a substrate.

A typical catalytic material for use on a filter member for dieselengine exhaust comprises platinum, palladium and rhodium dispersed on analumina and further comprises oxides of neodymium, strontium, lanthanum,barium and zirconium. Some suitable catalysts are described in U.S.patent application Ser. No. 08/761,544 filed Dec. 6, 1996, thedisclosure of which is incorporated herein by reference. In oneembodiment described therein, a catalytic material comprises a firstrefractory component and at least one first platinum group component,preferably a first palladium component and optionally, at least onefirst platinum group metal component other than palladium, an oxygenstorage component which is preferably in intimate contact with theplatinum group metal component in the first layer. An oxygen storagecomponent (“OSC”) effectively absorbs excess oxygen during periods oflean engine operation and releases oxygen during periods where localizedconcatenations of fuel produce a rich environment as seen in light-offof the catalyst after prolonged idle condition. Bulk ceria is known foruse as a OSC, but other rare earth oxides may be used as well. Inaddition, as indicated above, a co-formed rare earth oxide-zirconia maybe employed as a OSC. The co-formed rare earth oxide-zirconia may bemade by any suitable technique such as co-precipitation, co-gelling orthe like. One suitable technique for making a co-formed ceria-zirconiamaterial is illustrated in the article by Luccini, E., Mariani, S., andSbaizero, 0. (1989) “Preparation of Zirconia Cerium Carbonate in WaterWith Urea” Int. J. of Materials and Product Technology, vol. 4, no. 2,pp. 167-175, the disclosure of which is incorporated herein byreference. As disclosed starting at page 169 of the article, a dilute(0.1 M) distilled water solution of zirconyl chloride and cerium nitratein proportions to promote a final product of ZrO₂-10 mol % CeO₂ isprepared with ammonium nitrate as a buffer, to control pH. The solutionwas boiled with constant stirring for two hours and completeprecipitation was attained with the pH not exceeding 6.5 at any stage.

Any suitable technique for preparing the co-formed rare earthoxide-zirconia may be employed, provided that the resultant productcontains the rare earth oxide dispersed substantially throughout theentire zirconia matrix in the finished product, and not merely on thesurface of the zirconia particles or only within a surface layer,thereby leaving a substantial core of the zirconia matrix without rareearth oxide dispersed therein. Thus, co-precipitated zirconium andcerium (or one other rare earth metal) salts may include chlorides,sulfates, nitrates, acetates, etc. The co-precipitates may, afterwashing, be spray dried or freeze dried to remove water and thencalcined in air at about 500° C. to form the co-formed rare earthoxide-zirconia support. The catalytic materials of aforesaid applicationSer. No. 08/761,544 may also include a first zirconium component, atleast one first alkaline earth metal component, and at least one firstrare earth metal component selected from the group consisting oflanthanum metal components and neodymium metal components. The catalyticmaterial may also contain at least one alkaline earth metal componentand at least one rare earth component and, optionally, at least oneadditional platinum group metal component preferably selected from thegroup consisting of platinum, rhodium, ruthenium, and iridium componentswith preferred additional first layer platinum group metal componentsbeing selected from the group consisting of platinum and rhodium andmixtures thereof.

A variety of deposition methods are known in the art for depositingcatalytic material on a foraminous substrate. These methods of applyingthe catalytic component onto the substrate constitute a separate step inthe manufacturing process relative to the application of any anchorlayer (if applied) to the substrate.

Methods for depositing catalytic material on the foraminous substrateinclude, for example, disposing the catalytic material in a liquidvehicle to form a slurry and wetting the foraminous substrate with theslurry by dipping the substrate into the slurry, spraying the slurryonto the substrate, etc. Alternatively, the catalytic material may bedissolved in a solvent and the solvent may then be wetted onto thesurface of the foraminous substrate and thereafter removed to leave thecatalytic material, or a precursor thereof, on the foraminous substrate.The removal procedure may entail heating the wetted substrate and/orsubjecting the wetted substrate to a vacuum to remove the solvent viaevaporation.

EXAMPLE 1 Preparation of a Catalyst Composition Containing Platinum andPalladium in a 4:1 Ratio

First platinum and palladium compounds were dispersed on high surfacearea gamma alumina and 5% lanthanum modified alumina supports. Into2104.5 g of gamma alumina (97% solids) and 2041 g of 5% lanthanumstabilized alumina was added an aqueous solution containing 133.9 g ofPt as a 16% amine solubilized platinum hydroxide diluted with 709 g ofdeionized water with mixing. After mixing for additional 20 minutes a Pdsolution was added containing 33.5 g Pd as a 19% palladium nitratesolution diluted with 700 g of deionized water. This was mixed anadditional 20 minutes to ensure the powder was uniformly contacted withthe precious metal solution.

The resulting precious metal support mixture from above was contactedwith 6189 g of deionized water, 433.9 g of 90 acetic acid and 18 g ofoctanol in a dispersion tank. This mixture was fed into a continuousmill and ground until >90% of the material had a particle diameter ofless than 5 microns. A Ce/Zr composite oxide was added with anadditional 120 g of acetic acid and the resulting slurry was furtherground until the overall particle size was 90% <1 micron. In adispersion tank 583.3 g of zirconyl acetate solution was added to theslurry and mixed vigorously. The final pH of the slurry was in the rangeof 4.0-4.8.

EXAMPLE 2 Preparation of Wire Mesh Foraminous Catalytic Substrate

To prepare an article having the design as shown in FIG. 1, a stainlesssteel wool mesh was wire arc spray-coated with a nickel-aluminide alloyas described in Example 1 of the aforesaid '626 application. The steelwool mesh was then coated with the coating slurry described above(Example 1) at a washcoat loading of 0.05 to 0.1 g/in². The mesh wasthen fitted into a cylindrical channel having an inside diameter of 1.25in.

While this invention has been described with an emphasis upon preferredembodiments, it will be obvious to those of ordinary skill in the artthat variations in the preferred devices and methods may be used andthat it is intended that the invention may be practiced otherwise thanas specifically described herein. Accordingly, this invention includesall modifications encompassed within the spirit and scope of theinvention as defined by the claims that follow.

1. A filter for removing soot particles from the exhaust of a dieselengine comprising: an enclosed area, a plurality of channels disposedwithin said enclosed area and disposed longitudinally in the samedirection as gas flow through the filter, each of said channels having ahollow interior and containing opposed open ends, disposed within eachof said channels and substantially filling said hollow interior is anoptionally removable metal mesh.
 2. The filter of claim 1 including avoid space between said plurality of channels, said void space beingopen to said exhaust.
 3. The filter of claim 2 wherein the volume ofsaid void space is less than 25% of the volume of said enclosed area. 4.The filter of claim 1 wherein said metal mesh is a woven metal mesh. 5.The filter of claim 1 wherein said metal mesh is non-woven.
 6. Thefilter of claim 1 wherein said metal mesh contained in said channels isof a single mass.
 7. The filter of claim 1 wherein said channels arehollow cylinders.
 8. The filter of claim 1 wherein said metal mesh iscoated with an oxidation catalyst.
 9. The filter of claim 8 wherein saidoxidation catalyst is a platinum group metal.
 10. The filter of claim 8containing a metal anchor coat disposed between said metal mesh and saidcatalyst.
 11. The filter of claim 10 wherein said anchor coat is appliedby electric arc spraying.
 12. The filter of claim 8 wherein at least aportion of the interior surfaces of said channels are coated with anoxidation catalyst.
 13. The filter of claim 2 wherein said void space isdefined by a plurality of void spaces between each of said channels. 14.The filter of claim 13 wherein said channels are supported by opposedupstream and downstream end plates, said endplates containing aplurality of orifices to direct exhaust gas to said plurality of voidspaces.
 15. A method of removing soot from the exhaust gas of a dieselengine comprising directing said exhaust gas through a diesel filtercomprising an enclosed area, a plurality of channels disposed withinsaid enclosed area and disposed longitudinally in the same direction asgas flow through the filter, each of said channels having a hollowinterior end containing opposed open ends, disposed within each of saidchannels and substantially filling said hollow interior is an optionallyremovable metal mesh, whereby soot particles contained in said exhaustgas are trapped on said metal mesh.
 16. The method of claim 15 includinga void space between said plurality of channels, said void space beingopen to said exhaust gas flow.
 17. The method of claim 16 wherein thevolume of said void space is less than 25% of the volume of saidenclosed area.
 18. The method of claim 15 wherein said channels arehollow cylinders.
 19. The method of claim 15 wherein said metal mesh iscoated with an oxidation catalyst, and said trapped soot particles arecontinuously or periodically ignited and burned to regenerate thefilter.
 20. The method of claim 19 wherein said oxidation catalyst is aplatinum group metal.